Hello
As there is no "line in" facility on your keyboard, you won't be able to use its amp/speakers for Hauptwerk (other than by hacking into the keyboard to add line in). In any case, any reasonable active speakers would probably sound better.
I don't have experience of the M-Audio speakers you mention, but I would expect them to work well for you in a small church. However, you might like to consider getting something with a larger woofer.